There are two strategies you can adopt and some really good examples are provided in this stack link (How can I inject a property value into a Spring Bean which was configured using annotations?) .
Look at the answer from @Umut Utkan which describes the solution well.
Strategy 1: Add a method invoking factory bean and use that factory to add values to an existing map instead of erroring/overriding. However since you cant change the bean definition , i am not going in detail into this (The link has more info on how to achieve it)
Strategy 2 : Very simple, set the merge flag of the collection to true as below.
<bean id="columnMap" class="org.springframework.beans.factory.config.MapFactoryBean">
<property name="sourceMap">
<util:map>
<entry key="1" value="1"/>
</util:map>
</property>
</bean>
<bean id="specificColumnMap" class="org.springframework.beans.factory.config.MapFactoryBean" parent="columnMap">
<property name="sourceMap">
<util:map merge="true">
<entry key="2" value="2"/>
</util:map>
</property>
</bean>
PS : I dont know if this falls in the category of a duplicate question, so i am going to post the answer and let the moderators take a call on that.